Selective and sensitive fluorescent sensors for metal ions based on manipulation of side-chain compositions of poly(p-phenyleneethynylene)s

Article abstract of DOI:10.1021/ac049163m

The syntheses and metal-responsive properties of poly-(p- phenyleenethynylene)s with grafted new pseudo-crown-ether groups are reported. These polymers exhibit high sensitivities to alkali ions because of their collective optical properties, which are ver

Full text of DOI:10.1021/ac049163m

Anal. Chem. 2004, 76, 6513-6518
Correspondence
Selective and Sensitive Fluorescent Sensors for
Metal Ions Based on Manipulation of Side-Chain
Compositions of Poly(p-phenyleneethynylene)s
Zhen Chen,† Cuihua Xue,† Wei Shi,† Fen-Tair Luo,‡ Sarah Green,† Jian Chen,§ and Haiying Liu*,†
Department of Chemistry, Michigan Technological University, Houghton, Michigan 49931, Institute of Chemistry, Academia
Sinica, Taipei, Taiwan 115, Republic of China, and Zyvex Corporation, 1321 North Plano Road, Richardson, Texas 75081
sensing polymers include crown ether-substituted polypyrroles4
and polythiophenes,5 ethylene ether-substituted polythiophenes,6
macrocyclic ether-substituted polythiophene,7 and calix[4]arene-
based polythiophenes.8 Another sensing mechanism is based upon
electron density changes of polymer backbone in the presence
of metal ions, which triggers changes of UV or fluorescence
intensity of the polymers. These fluorescent sensing polymers
include 2,2′-bipyridyl-phenylene-vinylene-based polymers9 and
terpyridine-based poly(p-phenyleneethynylene)-alt-(thienylene-
ethynylene) polymers.10 The selectivities of the sensing polymers
are completely dependent upon the intrinsic binding properties
of recognition sites, which often lack selectivities and can respond
to different metal ions.
The syntheses and metal-responsive properties of poly-
(p-phenyleenethynylene)s with grafted new pseudo-crown-
ether groups are reported. These polymers exhibit high
sensitivities to alkali ions because of their collective
optical properties, which are very sensitive to ion-induced
conformational changes. The quenching of polymer fluo-
rescence caused by the conformational changes is pro-
portional to the ion concentration. The selectivity of the
sensing materials toward Li+ ions is significantly enhanced
by controlling the size of the binding site via manipulation
of the polymer side-chain compositions. The polymers are
very stable for their six-month solid-state storage at room
temperature.
With this limitation in mind, we report a novel approach to
enhance the selectivity of the sensing polymers by manipulating
side-chain compositions of poly(p-phenyleneethynylene)s. This
approach is illustrated in Chart 1. We introduce a new pseudo-
crown-ether to poly(p-phenyleneethynylene)s as polymer side
chains, which would undergo conformational changes upon
incorporating metal ions, resulting in attenuation of fluorescence
intensity. When phenyl groups in a polymer backbone that are
adjacent to those with the binding sites do not have any side
chains, polymer 1 lacks selectivity and can respond to alkali metal
ions such as lithium and sodium ions. However, when the adjacent
phenyl groups contain ethylene oxide side chains, polymer 2
displays a significantly enhanced selectivity and only responds to
lithium ions. In contrast, control polymers 3 and 4 with alkyl side
chains, instead of ethylene oxide side chains, display no response
to any metal ions because of lack of binding sites for metal ions.
Conjugated polymers have received considerable attention as
sensing materials because of their high sensitivities to analytes.
Their high sensitivities over devices using small molecules arise
from collective optical and conducting properties of the conjugated
polymers, which are extremely sensitive to minor external
structural perturbations or to electron density changes within the
polymer backbone in the presence of analytes.1-3 A variety of
conjugated polymers containing molecular recognition sites have
been reported as sensing materials for metal ions. One sensing
mechanism is based upon ion-induced conductivity changes,
arising from a decrease of charge carrier mobility or a change in
the polymer conjugation, that are measured via current or potential
changes by using cyclic voltammetry. These electrochemical
* To whom correspondence should be addressed. E-mail: hyliu@mtu.edu.
† Michigan Technological University.
‡ Academia Sinica.
§ Zyvex Corp.
(1) (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2 0 0 0 , 100,
2537-2574. (b) Swager, T. M. Acc. Chem. Res. 1 9 9 8 , 31, 201-207. (c)
Bunz, U. H. F. Chem. Rev. 2 0 0 0 , 100, 1605-644.
(4) Bartlett, P. N.; Benniston, A. C.; Chung, L.-Y.; Dawson, D. H.; Moore, P.
Electrochim. Acta 1 9 9 1 , 36, 1377-1379.
(5) (a) Bauerle, P.; Scheib, S. Adv. Mater. 1 9 9 3 , 5, 848-853. (b) Scheib, S.;
Ba¨uerle, P. J. Mater. Chem. 1 9 9 9 , 9, 2139-2150.
(2) (a) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1 9 9 8 , 120, 11864-11873.
(b) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2 0 0 3 ,
125, 896-900. (c) Jones, R. M.; Bergstedt, T. S.; McBranch, D. W.; Whitten,
D. G. J. Am. Chem. Soc. 2 0 0 1 , 123, 6726-6727.
(6) McCullough, R. D.; Williams, S. P. Chem. Mater. 1 9 9 5 , 7, 2001-2003.
(7) Marsella, M. J.; Swager, T. M. J. Am. Chem. Soc. 1 9 9 3 , 115, 12214-12215.
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Soc. 1 9 9 5 , 117, 9842-9848.
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DiCesare, N.; Pinto, M. R.; Schanze, K. S.; Lakowicz, J. R. Langmuir 2 0 0 2 ,
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10.1021/ac049163m CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/25/2004
Analytical Chemistry, Vol. 76, No. 21, November 1, 2004 6513

Chart 1. Chemical Structures of Poly(p-phenyleneethynylene)s
EXPERIMENTAL SECTION
through a 0.5-µm Fluoropore filter prior to use. The polymers were
1
Instrumentation. H NMR and 13C NMR spectra were taken
detected by a Waters model 440 ultraviolet absorbance detector
at a wavelength of 255 nm and a Waters model 2410 refractive
index detector. The polymer solutions were prepared at ∼1 mg/
mL concentration. Molecular weight was measured relative to
polystyrene standards.
on a 400-MHz Varian Unity Inova spectrophotometer in the
indicated solvents at the indicated fields. UV spectra were taken
on a Hewlett-Packard 8452A diode array UV-visible spectropho-
tometer. Fluorescence spectra were obtained on a steady-state
1681 Spex Fluorolog fluorometer at room temperature. Fluores-
cence measurements were performed in 3.3 × 10-8 and 5.2 ×
10-8 M chloroform solutions for polymers 1 and 2 , respectively.
The polymer chloroform solutions with different metal ion
concentrations were prepared for fluorescent measurements of
the metal ions. Molecular weights of the polymers were deter-
mined by gel permeation chromatography by using a Waters
Associates model 6000A liquid chromatograph. Three American
Polymer Standards Corp. Ultrastyragel columns in series with
porosity indexes of 103, 104, and 105 Å were used and housed in
an oven thermostated at 30 °C. Mobile phase was HPLC grade
THF, which was filtered and degassed by vacuum filtration
Materials. Unless otherwise indicated, all reagents and
solvents were obtained from commercial suppliers (Aldrich, Fluka,
Acros, Lancaster) and were used without further purification. Air-
and moisture-sensitive reactions were conducted in oven-dried
glassware using standard Schlenk line or drybox techniques under
an inert atmosphere of dry argon. 1,4-Diethynylbenzene, 1,4-bis-
(dodecyloxy-2,5-diethynyl)benzene, and 1,4-bis((triethylene glycol
monomethyl ether)oxy)-2,5-diethynylbenzene were prepared and
characterized according to previous procedures.11 Toxic chemicals
(11) Xue, C.; Chen, Z.; Wen, Y.; Shi, W.; Luo, F.-T.; Chen, J.; Liu, H. Submitted
to Langmuir.
6514 Analytical Chemistry, Vol. 76, No. 21, November 1, 2004

Scheme 1
Scheme 2
such as oxalyl chloride were handled in a fume hood by wearing
gloves.
Compound 6 (Scheme 1 ). To a dried round-bottomed flask
were added 5[4-(4-carboxybutoxy)-2,5-diiodophenoxy]pentanoic
acid (2.0 g, 3.56 mmoL), CH2Cl2 (60 mL), and oxalyl chloride (6
mL, 69.8 mmoL), and then a catalytic amount of DMF was added.
The mixture was stirred at room temperature for 6 h until the
solid disappeared. The solvent and excess oxalyl chloride was
removed under reduced pressure to give the 5 as a faint yellow
solid. Without further purification, 5 was dissolved in the dried
CH2Cl2 (20 mL) and then added dropwise to the solution of bis-
(2-(2-(2-methoxyethoxy)ethoxy)ethyl)5-aminoisophthalate (4) (3.54
g, 7.48 mmol) in dried CH2Cl2 (20 mL) and pyridine (1.0 mL, 12.4
mmol). The reaction mixture was stirred at room temperature
for 8 h. The solvent was removed, and the residue was diluted
with ethyl acetate (50 mL), washed with 10% NaHCO3 (30 mL ×
3) and brine (30 mL × 3), and dried over anhydrous MgSO4. The
solvent was evaporated, and the crude compound was purified
by column chromatography on silica gel with EtOAc/ acetone (5:
1, V/ V) and then recrystallization from EtOAc/ hexane to give the
target compound 6 (4.0 g, 76% yield) as a white solid: 1H NMR
(ppm, DMSO-d6) 10.35 (s, 2H), 8.48 (s, 4H), 8.12 (s, 2H), 7.29 (s,
2H), 4.40 (m, 8H), 3.96 (m, 4H), 3.73 (m, 8H), 3.55 (m, 8H), 3.44-
3.50 (m, 16H), 3.33-3.35 (m, 8H), 3.15 (s, 12H), 2.39 (m, 4H),
1.76 (m, 8H). 13C NMR (ppm, CDCl3) 22.40, 28.70, 37.02, 59.18,
64.73, 69.26, 69.94, 70.79, 70.84, 70.94, 72.10, 86.48, 122.84, 124.95,
126.30, 131.34, 138.96, 152.83, 165.60; HRMS, Calcd 1473.3738 for
[M + H]+ (C60H87I2N2O24). Found 1473.3738.
phenyl)methanol, dried THF (30 mL), and pyridine (2.5 mL). The
reaction mixture was stirred at 40 °C for 8 h. The solvent was
removed, and the residue was diluted with ethyl acetate (50 mL),
washed with 10% NaHCO3 (30 mL × 3) and brine (30 mL × 3),
and dried over anhydrous MgSO4. The solvent was evaporated,
and the crude compound was purified by column chromatography
on silica gel with hexanes/ EtOAc (10:1, v/ v) to give the target
compound 8 (9.34 g, 71% yield) as a faint yellow oil: 1H NMR
(ppm, CDCl3) 7.14 (s, 2H), 6.45 (s, 4H), 6.37 (s, 2H), 5.02 (s, 4H),
4.05 (t, J ) 7.0 Hz, 4H), 3.88-3.93 (m, 8H), 2.46 (t, J ) 7.0 Hz,
4H), 1.83-1.88 (m, 8H), 1.70-1.77 (m, 8H), 1.52 (m, 8H), 1.38-
1.41 (m, 8H), 1.24-1.28 (m, 56H), 0.86 (t, J ) 7.0 Hz, 12H); 13C
NMR (ppm, CDCl3) 14.42, 22.01, 22.98, 26.35, 28.90, 29.55, 29.64,
29.70, 29.88, 29.93, 29.96, 32.21, 34.17, 64.87, 66.50, 68.39, 70.03,
86.56, 101.26, 106.69, 123.08, 138.32, 153.12, 160.76, 173.46. HRMS,
Calcd 1478.7623 (C78H128I2O10). Found 1478.7622.
P olymer 1. Monomer 6 (1.0 equiv), 1,4-diethynylbenzene (1.1
equiv), and iodobenzene as polymer end groups (0.1 equiv) were
polymerized for polymer 1 by using the Sonogashira cross-
coupling reaction in the presence of 5% (PPh3)4Pd and 5% CuI in
DMF and diisopropylamine at 60 °C for 2 days. The iodobenzene
(0.3 equiv) was added to the mixture, which reacted for another
3 h. The polymer was precipitated in ethanol, filtered, washed
Compound 8 (Scheme 2 ). 5-[4-(4-Carboxybutoxy)-2,5-di-
iodophenoxy]pentanoic acid (5.0 g, 8.90 mmoL) in CH2Cl2 (60 mL)
was reacted with oxalyl chloride (8 mL, 93.10 mmol) in the
presence of a catalytic amount of DMF to prepare 5 according to
above procedure. Compound 5 was dissolved in dried CH2Cl2 (40
mL) and added dropwise to the mixture of (3,5-bis(dodecyloxy)-
Analytical Chemistry, Vol. 76, No. 21, November 1, 2004 6515

with acetone, and dried under vacuum at room temperature: 1H
NMR (ppm, DMSO-d6) 10.38 (s, 2H), 8.50 (s, 4H), 8.14 (s, 2H),
7.55 (d, 4H), 7.19 (s, 2H), 4.39 (m, 8H), 4.10 (m, 4H), 3.70 (m,
8H), 3.54 (m, 8H), 3.45 (m, 16H), 3.33 (m, 8H), 3.15 (s, 12H),
2.39 (m, 4H), 1.85 (m, 8H). Gel permeation chromatography
analysis (mobile phase: THF, polystyrene standards) indicates
that Mw of the polymer is 75 633, and its polydispersity is 2.56.
P olymer 2 . Monomer 6 and 1,4-bis((triethylene glycol mono-
methyl ether)oxy)-2,5-diethynylbenzene were polymerized in a way
similar to that for polymer 1 : 1H NMR (ppm, DMSO-d6) 10.35
(s, 2H), 8.48 (s, 4H), 8.15 (s, 2H), 7.11 (s, 4H), 4.41 (m, 8H), 4.17
(m, 8H), 3.73 (m, 12H), 3.55 (m, 12H), 3.47 (m, 24H), 3.35 (m,
12H), 3.17 (s, 18H), 2.45 (m, 4H), 1.83 (m, 8H). Mw of the polymer
is 29 447, and its polydispersity is 1.93.
P olymer 3 . Monomer 8 and 1,4-diethynylbenzene were
polymerized to give polymer 3 in a way similar to that for polymer
1 except using toluene as reaction solvent: 1H NMR (ppm, CDCl3)
7.47 (d, J ) 6.4 Hz 4H), 6.98 (s, 2H), 6.44 (s, 4H), 6.37 (s, 2H),
5.00 (s, 4H), 4.00 (t, 4H), 3.91-3.86 (m, 8H), 2.49 (t, 4H), 1.91
(m, 8H), 1.74-1.69 (m, 8H), 1.39 (m, 16H), 1.23 (m, 56H), 0.85
(t, J ) 7.0 Hz, 12H). Mw of the polymer is 129 945, and its
polydispersity is 4.35.
Figure 1. Fluorescence spectra of 3.3 × 10-8 M polymer 1 in
chloroform solutions with different metal ions at an excitation wave-
length of 405 nm.
THF, and toluene, but insoluble in acetone, ethanol, DMF, DMSO,
and water. The UV-visible absorption and fluorescence spectra
of the polymers were recorded in their dilute CHCl3 solution at
room temperature. Polymer 1 displays absorption maximum at
422 nm and emission maximum at 448 nm, which were ascribed
to the π-π transition of the conjugated polymer backbone.
Polymer 2 exhibits absorption maximum at 442 nm, emission
maximum at 472 nm, significant red shifts in its absorption and
emission compared with that of polymer 1 . These red shifts are
due to an electron-donating effect from the dialkoxy groups.
Control polymer 3 exhibits maximum absorption and emission
at 414 and 445 nm, and polymer 4 shows maximum absorption
and emission at 438 and 473 nm, respectively. These red shifts in
absorption and emission spectra of the polymer 4 also result from
the electron-donating effect of the dialkoxy groups compared with
the polymer 3 . All fluorescence spectra were obtained by using
excitation wavelengths at 405 and 420 nm for polymers 1 (see
Figure 1) and 2 , respectively. Use of shorter excitation wave-
lengths does not shift the maximum emission wavelength of the
polymers but decreases their fluorescence intensity. All polymers
obey Beer-Lambert’s law in their dilute chloroform solutions. The
molar absorption coefficients of the polymers 1 and 2 are 2.99 ×
106 and 2.17 × 106 L mol-1 cm-1, and those of polymers 3 and 4
are 8.69 × 106 and 2.34 × 106 L mol-1 cm-1, respectively. The
higher molecular weight polymer exhibits the larger molar
absorption coefficients because of the more enhanced conjugation
with the higher molecular weight.
Ion-responsive properties of the polymers 1 and 2 were
evaluated by their emission spectra. Some representative results
for the polymer 1 are shown in Figure 1. It is apparent that
addition of metal ions causes quenching of the polymer 1
fluorescence, which presumably comes from conformational
changes of the polymer backbone when the binding sites
incorporate metal ions. Control experiments using polymers 3
and 4 (lacking metal-binding sites) in lieu of polymers 1 and 2
exhibit no quenching of polymer fluorescence for all metal ions
tested. This finding confirms that the pseudo-crown-ether side
chains are vital for ion recognition in the polymers 1 and 2 .
There is a slight decrease in UV-visible spectra of the polymer
upon addition of LiPF6 or NaPF6 to the polymer 1 chloroform
solution. The UV-visible spectra of the polymer remain essentially
Polymer 4 : Monomer 8 and 1,4-bis-dodecyloxy-2,5-diethynyl-
benzene were polymerized to give polymer 4 in a similar way for
1
polymer 3 . H NMR (ppm, CDCl3): 6.96 (s, 4H), 6.42 (s, 4H),
6.36 (s, 2H), 4.97 (s, 4H), 3.97-3.89 (m, 16H), 2.45 (t, 4H), 1.88-
1.73 (m, 20H), 1.45-1.23 (m, 108H), 0.85 (t, 18H). Mw of the
polymer is 37,065 and its polydispersity is 2.12.
RESULTS AND DISCUSSION
To allow for the coupling of pseudo-crown-ether groups to the
backbone of poly(p-phenyleneethynylene)s, we developed a syn-
thesis strategy based on the use of the intermediate, 5[4-(4-
chlorocarbonylbutoxy)-2,5-diiodophenoxy]pentanoyl chloride (5 ).
Monomers 6 and 8 were obtained by coupling the intermediate
with bis(2-(2-(2-methoxyethoxy)ethoxy)ethyl)5-aminoisophthalate
(4 ) and 3,5-didodecyloxybenzyl alcohol (7 ) in THF solution in the
presence of pyridine, respectively. Poly(p-phenyleneethynylene)s
are typically synthesized by a cross-coupling polymerization under
Sonogashira conditions of a diiodoaryl monomer with a diethynyl
monomer. Polymer 1 synthesized by cross-coupling of 1,4-
diethynylbenzene and monomer 6 has a high molecular weight
of 75 633 and is readily soluble in common organic solvents such
as CH2Cl2, CHCl3, THF, DMF, DMSO, and toluene. Polymer 2 ,
synthesized by cross-coupling of 1,4-bis((triethylene glycol mono-
methyl ether)oxy)-2,5-diethynylbenzene and monomer 6 , has a
little lower molecular weight of 29 447 compared with polymer 1
because steric hindrance from the (triethylene glycol monomethy
ether)oxy groups slows down the polymerization. Polymer 2 is
also readily soluble in common organic solvents such as CH2Cl2,
CHCl3, THF, DMF, DMSO, and toluene, but insoluble in ethanol,
acetone, and water. For comparison purposes, polymers 3 and 4
were synthesized from monomer 8 with 1,4-diethynylbenzene and
1,4-bis(dodecyloxy-2,5-diethynyl)benzene, respectively, by using
the same conditions for polymer 1 except using toluene as the
reaction solvent. Similar steric hindrance from dodecyloxy groups
leads to a little lower molecular weight for polymer 4 (Mw 37 065)
in comparison with polymer 3 (Mw 129 945). Polymers 3 and 4
are readily soluble in common solvents such as CH2Cl2, CHCl3,
6516 Analytical Chemistry, Vol. 76, No. 21, November 1, 2004

Figure 2. Changes in the emission intensity of 3.3 × 10-8
M
Figure 3. Fluorescence spectra of 5.2 × 10-8 M polymer 2 in
chloroform solutions with different metal ions at an excitation wave-
length of 420 nm.
polymer 1 in chloroform solutions as a function of added different
metal ions at an excitation wavelength of 405 nm.
identical upon addition of KPF6 to the polymer 1 chloroform
solution. The UV-visible spectra are significantly less responsive
to the polymer conformational change caused by the polymer
binding with the metal ions. However, addition of 10 mM Li+ or
Na+ ions to the polymer 1 solution causes significant attenuation
of the polymer fluorescence intensity (Figure 1). Polymer 1 has
larger responses to Li+ and Na+ ions, which reflects a better size
match with the binding sites. It displays the least sensitivity to
K+ ions because of its larger size. Moreover, polymer 1 also
exhibits linear responses to Li+ or Na+ ions (Figure 2). The
detection limits for Li+ and Na+ ions are 4.0 × 10-5 and 6.0 ×
10-5 M, respectively. Nevertheless, it is apparent that polymer 1
lacks adequate selectivity and responds to different metal ions
because of the relative flexibility of its binding size. As a result, it
is difficult to calibrate the response of polymer 1 to Li+ ions in
the presence of different metal ions such as Na+ ions. With this
limitation in mind, we present a novel approach and demonstrate
the feasibility to control the binding size and enhance selectivity
toward metal ions by manipulating polymer side-chain composi-
tions. Polymer 2 was prepared by introducing ethylene oxide side
chains to phenylene groups in the polymer backbone, adjacent
to those with the binding sites. Addition of LiPF6 to the polymer
2 chloroform solution causes a slight decrease in UV-visible
spectra of the polymer while the UV-visible spectra of the
polymer remain essentially identical upon addition of NaPF6 or
KPF6 to the polymer 2 chloroform solution. However, addition of
Li+ ions to the polymer solution significantly reduces the polymer
fluorescence intensity (Figure 3). Polymer 2 displays a signifi-
cantly reduced response to Na+ ions, compared with polymer 1
while retaining its high sensitivity to Li+ ions (Figure 3). This
enhanced selectivity of polymer 2 occurs because the ethylene
oxide side chains limit the expansion of the binding size, which
is a best size match for Li+ ions. Moreover, the enhanced selectiv-
ity does not affect linear response of polymer 2 to Li+ ions
(Figure 4). The tolerance level for selective determination of Li+
ions at its detection limit of 4.0 × 10-5 M in the mixture of
alkali ions is that the concentration of Na+ ions is less than 0.5
mM. This tolerance level is completely dependent on Li+ ion
concentration. The level increases with an increase of Li+ ion
concentration.
Figure 4. Changes in the emission intensity of 5.2 × 10-8
M
polymer 2 in chloroform solutions as a function of added different
metal ions at an excitation wavelength of 420 nm.
To test ion binding reversibility of polymers 1 and 2 , we
recycled the polymers by washing the polymer chloroform
solutions containing metal ions with distilled water (100 mL ×
10), removing the solvent, washing the polymers with ethanol and
acetone completely, and drying them under vacuum at room
temperature. The recycled polymer 1 or 2 in chloroform solution
shows similar responses to the metal ions, which indicates that
the bindings of the polymers with the metal ions are somewhat
reversible. This good feature allows for potential preparation of
reusable fiber-optic sensors for metal ions by immobilizing the
polymers on optic fibers, whose binding ions could be washed
away with water.
Polymers 1 and 2 are very stable during solid storage since
they display almost the same responses to the metal ions after
six-month storage at room temperature. Polymers 1 and 2 exhibit
almost the same responses to Li+ or Na+ ions when they were
stored in chloroform solution for two months.
We have designed and synthesized poly(p-phenyleneethy-
nylene)s with a new pseudo-crown-ether as a binding site for alkali
Analytical Chemistry, Vol. 76, No. 21, November 1, 2004 6517

ions. The sensitivity of these polymers results from ion recogni-
tion-induced conformational changes on the polymer backbones,
which lead to quenching of polymer fluorescence. The selectivity
of the sensing material to Li+ ions is significantly enhanced by
controlling polymer side-chain compositions to manipulate the
size of binding site.
David Waldeck for insightful discussion and reviewers for their
valuable comments.
SUPPORTING INFORMATION AVAILABLE
Preparation of starting materials and UV-visible and fluores-
cence spectra of 1 -4 . This material is available free of charge
via the Internet at http:/ / pubs.acs.org.
ACKNOWLEDGMENT
Received for review June 8, 2004. Accepted August 27,
2004.
We acknowledge startup fund, research excellent fund of
Michigan Tech, NSF-NER, Zyvex Corp., and NASA (Grant
NNJ04JA18C) for support of this work. We also thank Professor
AC049163M
6518 Analytical Chemistry, Vol. 76, No. 21, November 1, 2004